Rolling bearing for hard disk drive

ABSTRACT

A rolling bearing for an HDD being excellent in the resistance to fretting wear, capable of lowering the torque and relieving the torque change and having a long life while exerting no undesirable effect on various bearing properties. A rolling bearing for hard disk drives wherein a cage  7  supports a plural number of rolling elements  5  located between an inner ring  2  and an outer ring  4  and sealing members  6,6  fixed to the both ends in the axial direction of one of said inner ring  2  and said outer ring  4  are opposite to each other, wherein a lubricating oil has been injected into the bearing space formed between said sealing members  6,6  at the both ends in the axial direction so as to amount to 1 to 50% by volume of the bearing space.

BACKGROUND OF THE INVENTION

This invention relates to rolling bearings for hard disk drives(hereinafter referred to simply as HDD), in particular, rolling bearingsappropriately employed in HDD components with rocking motion such asactuators (in particular, swing arms) or high-speed rotary motion suchas spindles.

The present application is based on Japanese Patent Application No. Hei.9-149403, which is incorporated herein by reference.

Although the computer industry has a short history compared with otherindustries, rapid technical innovation has been progressing in thisfield. Among all, HDDs undergo quick model changes and there have beendeveloped novel ones for saving electricity, establishing quick responseand high accuracy, downsizing, etc.

Under these circumstances, it has been required to improve theproperties of bearings to be integrated into HDDs and prolonging thelife thereof. For example, rolling bearings to be used in componentswith reciprocating rocking motion (for example, swing arm) should behighly resistant to fretting wear and have low torque and long life.Similarly, rolling bearings to be used in components with high-speedrotary motion (for example, spindle) should be excellent in heatresistance and high-speed rotary performance and have long life.

In a rolling bearing to be put into a swing arm, a spindle, etc. asshown in FIG. 1, for example, an inner ring 2 having the inner racewaysurface 1 in its outer periphery and an outer ring 4 having the outerraceway surface 3 in its inner periphery are located concentrically anda plural number of balls 5,5 as rolling elements provided between theinner raceway surface 1 of the inner ring and the outer raceway surface3 of the outer ring are supported by a cage 7 in such a manner as toallow free rolling. Circular sealing members 6,6 fixed to the both endsin the axial direction of the outer ring 4 prevent a grease lubricatingoil (not shown in the figure), which has been injected into the bearingsurface formed between the sealing members 6,6 from leakage. Thesesealing members also prevent the invasion of dusts suspending outside.

Although the grease lubrication is mainly employed for lubricatingbearings, a rust proof lubricating oil is applied to the bearing racewaysurfaces 1, 3, etc. prior to the injection of a grease in some cases. Inthe latter lubricating method, the initial lubrication is carried out byusing not the grease but the lubricating oil so as to improve thelubricating performance.

Rotation is repeated at a minor rocking angle (usually 26° or below) ina swing arm rolling bearing. In the lubricating method with the use of agrease as described above, however, it is frequently observed that thelubricant masses are scraped out from the contact face, and also thereis a possibility that the lubricant masses cause insufficientlubrication locally. This local insufficient lubrication inducesfretting wear, thus shortening the life of the bearing. Moreover, therunning torque at a minor angle is increased by the stirring resistanceof the grease or the hang-up of the grease changes the torque. Thesephenomena lower the reliability of the HDD in reading and writing athigh accuracy.

In a spindle bearing, use of a grease results in similar troubles suchas an increase in running torque and changes in torque. In particular,the high-speed rotary performance is seriously affected thereby.

To solve these problems, attempts have been made to prevent the increasein torque and relieve the change therein by reducing the amount of thegrease to be injected. When the grease is used only in an insufficientamount, however, there is a possibility that a fretting wear is arisenand the lubricating effect disappears earlier, which are not favorablefrom the viewpoint of the bearing life.

When the grease is injected in an increased amount, on the other hand,the increase and change in torque can be hardly regulated, though thefretting wear can be prevented and the bearing life can be prolongedthereby.

In the method wherein a rust proof lubricating oil is used together witha grease, the lubricating properties at the early stage can beparticularly improved. However, this method essentially comprisesinjecting the grease. Therefore, the problems in association with greasecannot be fundamentally solved thereby.

In addition, it has been a practice to apply a rust proof lubricatingoil onto a bearing by immersing the bearing in the rust prooflubricating oil. As a result, a large amount of the rust prooflubricating oil adheres to the outer periphery of the bearing, whichdoes not directly participate in the lubricating effect, and makes itsticky and tacky. Before introducing into a HDD component (for example,swing arm unit or spindle motor), such a bearing should be cleansed bywiping, which brings about a decrease in the productivity and anincrease in the production cost. Moreover, it is feared that theremaining lubricating oil might volatilize and stain recording media inthe HDD.

As described above, there have been desired rolling bearings, which arecomponents of HDDs aiming at saving electricity, establishing quickresponse and high accuracy, downsizing, etc., having high resistance tofretting wear, low torque, relieved torque change and long life.However, these requirements cannot be satisfied by using the existinggrease lubricating method or the one with the combined use of a greasewith a rust proof lubricating oil.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide arolling bearing for an HDD being excellent in the resistance to frettingwear, capable of lowering the torque and relieving the torque change andhaving a long life while exerting no undesirable effect on variousbearing properties.

The above-mentioned object of the present invention can be achieved by arolling bearing for hard disk drives wherein a cage supports a pluralnumber of rolling elements located between an inner ring and an outerring and sealing members fixed to the both ends in the axial directionof one of said inner ring and said outer ring are opposite to eachother, characterized in that a lubricating oil has been injected intothe bearing space formed between said sealing members at the both endsin the axial direction so as to amount to 1 to 50% by volume of thebearing space.

In the rolling bearing for HDDs of the present invention, films made ofa lubricating oil are formed, as a substitute for the greases employedin the conventional cases, on the surface of the inner ring and outerring raceway surfaces, rolling elements and cages to achieve lubricatingeffect. Such a surface coated with the lubricating oil film will behereinafter called “lubricant face”. Compared with greases, lubricatingoils are highly flowable and free from any increase or change in torqueas observed in the case with the use of greases. Thus, the rocking orhigh-rotary motion can be facilitated thereby. In addition, there arisesno problem of the local insufficient lubrication due to the lubricantmasses scraped out from the contact face of the inner ring and outerring raceway surfaces and rolling elements, thus establishing a highresistance to fretting wear.

Since the lubricating oil is injected into the bearing space to as toamount to 1 to 50% by volume of the bearing space, the lubricatingeffect would not disappear early.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a sectional view of an example of a rolling bearing for HDDs.

DETAILED DESCRIPTION OF THE INVENTION

Now, the rolling bearing for HDDs of the present invention will bedescribed in detail.

The rolling bearing for HDDs of the present invention is characterizedin that a lubricating oil is injected, as a substitute for theconventional greases, into the bearing space of a rolling bearing forHDDs as shown in FIG. 1. The bearing space between the sealing members6,6 at the both ends in the axial direction means the space between thesealing members 6,6 at the both ends in the axial direction of the innerring 2 or the outer ring 4. Thus, the space among the rolling balls 5and the cage 7 is excluded therefrom.

It is preferable that the lubricating oil is injected into the bearingspace so as to amount to 1 to 50% by volume of the space. When thecontent of the lubricating oil is less than 1% by volume, there arisesome problems, for example, insufficient thickness of the lubricatingoil film or early dry-up of the lubricating oil. When the lubricatingoil is injected in an amount exceeding to 50% by volume, on the otherhand, the lubricating oil frequently leaks out, though the lubricatinglife can be prolonged in this case.

It is desirable that the amount of the lubricating oil to be injected isappropriately determined within the range as defined above depending onthe component into which the bearing is to be put.

In a rolling bearing employed in a high-speed rotary component such as aspindle, from among those to be put into HDDs, namely, it is preferableto inject the lubricating oil in a larger amount from the viewpoints ofthe heat resistance and high-speed rotary performance of the outer ring4. In this case, it is preferable to inject the lubricating oil in alarge amount up to 50% by volume. In a rolling bearing employed in acomponent with reciprocating rocking motion such as a swing arm, on theother hand, excessively high heat resistance and high-speed rotaryperformance are unnecessary. In this case, it is enough to inject thelubricating oil in an amount of 30% by volume or less, preferably from 4to 25% by volume.

Although the method of injecting the lubricating oil is not particularlyrestricted, it is needed to utilize a method capable of controlling theamount of the injection. For example, the sealing member 6 ispreliminarily put into the lower part of the bearing and then fixed toone end in the axial direction of the outer ring 4. Next, nozzles 8, inthe same number as that of the pockets in the cage 7, are put downwardthereon and the lubricating oil is uniformly injected from the nozzlesin a predetermined amount into the bearing. Then the sealing member 6 isput in and fixed to another end of the outer ring 4.

By using this injection method, a large amount of the lubricating oilcan be encapsulated in the position to be lubricated in the bearing,thus ensuring the formation of thick lubricating oil films, comparedwith the conventional method wherein a bearing is immersed in alubricating oil. Further, the lubricating oil does not adhere to theouter surface of the bearing and, therefore, no procedure for wiping thelubricating oil is needed, different from the conventional methodwherein a bearing is immersed in a lubricating oil. Thus the workingefficiency can be highly elevated. Since the outside of the bearing isabsolute dry, there is no fear that the lubricating oil might stainrecording media in the HDD.

When the cage 7 preliminarily containing the lubricating oil is put intothe bearing, the lubricating oil would ooze out onto the surface of thecage 7 as the bearing rotates. Thus the lubricating oil is continuouslysupplied to the lubricant face for a long time and thus the lubricatinglife is prolonged.

It is preferable to determine the content of the lubricating oil in thecage 7 depending on the component into which the bearing is to be put.The lubricating oil contained in the cage 7 is excluded from thelubricating oil to be injected into the bearing space. The lubricatinglife can be prolonged while the mechanical strength of the cage islowered with an increase in the lubricating oil content in the cage 7.In the case of a rolling bearing to be used in a high-speed rotarycomponent such as a spindle wherein mechanical strength of a certaindegree is needed, it is therefore preferable to maintain the mechanicalstrength at the desired level by regulating the lubricating oil content.In such a case, it is preferable that the upper limit of the lubricatingoil content is 40% by weight based on the weight of the cage 7. In thecase of a rolling bearing to be used in a reciprocating rockingcomponent such as a swing arm wherein the mechanical strength may not beso high, the lubricating oil content can be elevated so as to prolongthe lubricating life. In such a case, the lubricating oil content may beup to 80% by weight, preferably from 10 to 70% by weight. In each case,the lower limit of the lubricating oil content is 0.1% by weight. Whenthe lubricating oil content is less than 0.1% by weight, the lubricatingoil is supplied from the cage 7 only in an excessively small amount andthus cannot contribute to the lubrication of the lubricant faces.

The lubricating oil may be introduced into the cage 7 by immersing thecage 7 in the lubricating oil, kneading a resin together with thelubricating oil followed by molding, etc. When the lubricating oilcontent is 3% by weight or less, it is convenient to immerse the cage 7in the lubricating oil. When the lubricating oil is to be introducedthereinto in a larger amount, it is convenient to use the latter method.In this case, it should be taken into consideration that the capabilityof the resin, of which the cage 7 is made, of holding the lubricatingoil varies depending on the type of the resin or the combination of theresin with the lubricating oil.

The sealing members 6,6 fixed to the both ends in the axial direction ofthe inner ring 2 may be opposite to the outer ring 4. Either a contactseal or a non-contact one may be used as the sealing members 6,6. Therolling elements may be rollers.

Although the lubricating oil is not particularly restricted, use oflubricating oils composed of the base oils with various additives aswill be described hereinbelow makes it possible to obtain lubricantcompositions being excellent in fretting wear resistance, heatresistance and high-speed rotary performance, thus achieving the objectof the present invention.

When the lubricating properties and heat resistance and solubility ofadditives are taken into consideration, it is preferable that the baseoil contain ester oils. Although the ester oils are not particularlyrestricted, preferable examples thereof include diester oils obtained byreacting a dibasic acid with a branched alcohol, aromatic ester oilsobtained by reacting an aromatic acid basic acid with a branched alcoholand hindered ester oils obtained by reacting a polyhydric alcohol with amonobasic acid. From the viewpoint of low volatility for preventingrecording media in HDDs from stains, it is preferable to use oneselected from among aromatic ester oils, hindered ester oils andmixtures thereof.

Examples of the diester oils include dioctyl adipate (DOA), diisobutyladipate (DIBA), dibutyl adipate (DBA), dioctyl azelate (DOZ), dibutylsebacate (DBS), dioctyl sebacate (DOS) and methyl acetyl ricinoleate(MAR-N).

Examples of the aromatic ester oils include trimellitate,pyromellitiate, trioctyltrimellitate (TOTM), tridecyltrimellitate andtetraoctylpyromellitate.

Examples of the hindered ester oils include those obtained by reacting apolyhydric alcohol with a monobasic acid, each as will be shownhereinbelow. Either one or more monobasic acids may be reacted with apolyhydric alcohol. Also, use may be made of complex esters which areoligoesters of polyhydric alcohols with mixed fatty acids comprisingdibasic acids with monobasic acids.

Examples of the polyhydric alcohols include trimethylolpropane (TMP),pentaerythritol (PE), dipentadrythritol (DPE), neopentyl glycol (NPG)and 2-methyl-2-propyl-1,3-propane (MPPD).

As the monobasic acids, monovalent C₄₋₁₈ fatty acids are mainlyemployed. Particular examples thereof include acetic acid, valerianicacid, caproic acid, caprylic acid, enanthic acid, pelargonic acid,undecanoic acid, lauric acid, caprylic acid, myristic acid, palmiticacid, beef tallow fatty acids, stearic acid, caproleic acid, undecylenicacid, linderic acid, tsuzuic acid, physeteric acid, myristoleic acid,palmitoleic acid, petroselinic acid, oleic acid, elaidic acid,asclepinic acid, vaccenic acid, sorbic acid, linolic acid, linolenicacid, sabinic acid and ricinoleic acid.

Such ester oil amounts at least to 20% by weight of the base oil. Whenthe lubricating properties are taken into consideration, it ispreferable that the ester oil contains at least 40% by weight ofpentaerythritol ester, dipentaerythritol ester or a mixture thereof.

When the content of the ester oil is less than 20% by weight, nosufficient fretting wear resistance (lubricating properties) can beachieved. The upper limit thereof is not particularly determined.

In addition to the above-mentioned ester oil, the base oil may furthercontain synthetic hydrocarbon oils, ether oils and mineral oils.

Examples of the synthetic hydrocarbon oils include poly-α-olefin oilsand α-olefin/ethylene cooligomers.

Examples of the ether oils include phenyl ether oils obtained byintroducing C₁₂₋₂₀ (di)alkyl chains into diphenyl, triphenyl ortetraphenyl. From the viewpoint of low volatility, (di)alkyl polyphenylether oils are preferable. Based on the specification on the above esteroil, these oils are contained in an amount of not more than 80% byweight.

The base oil should have a kinematic viscosity of at least 30 mm²/sunder a temperature of 40° C. When its kinematic viscosity is less than30 mm²/s, sufficient film formation cannot be achieved during rotationand thus the bearing life is shortened. Although the upper limit of thekinematic viscosity is not particularly specified, the kinematicviscosity is to be not more than 400 mm²/s, by considering the handlingproperties, oil film formation and increase in torque. To form an oilfilm having a sufficient fretting wear resistance, it is preferable thatthe kinematic viscosity ranges from 40 to 200 mm²/s.

The durability of the lubricating oil film can be improved by addingrust proof agents, oily components, antioxidants, etc. thereto.

As the rust proof agents, it is preferable to use organic sulfonic acidmetal salts or esters. Examples of the organic sulfonic acids includedinonylnapthalenesulfonic acid and heavy alkylbenzenesulfonic acids.Examples of metal salts thereof include calcium sulfonate, bariumsulfonate and sodium sulfonate.

Examples of the sorbitan derivatives as esters include partial esters ofpolybasic carboxylic acids and polyhydric alcohols such as sorbitanmonolaurate, sorbitan tristearate, sorbitan monooleate and sorbitantrioleate. Examples of the alkyl esters include polyoxyethylene laurate,polyoxyethylene oleate and polyoxyethylene stearate.

As the rust proof agent, use can be made of these organic sulfonic acidmetal salts and esters either alone or as a mixture thereof.

Preferable examples of the oily components include higher fatty acids(oleic acid, stearic acid, etc.), higher alcohols (lauryl alcohol, oleylalcohol, etc.), amines (stearylamine, cetylamine, etc.) and phosphates(tricresyl phosphate, etc.). These compounds may be employed eitheralone or as a mixture thereof.

As the antioxidants, it is preferable to use a mixture of anitrogen-containing compound-based antioxidant with a phenol antioxidantor sulfur-based antioxidants.

Examples of the nitrogen-containing compound-based antioxidant includephenyl-α-naphthylamine, diphenylamine, phenylenediamine, oleylamidoamineand phenothiazine.

Examples of the phenol antioxidant include hindered phenols such asp-t-butyl phenyl salicylate, 2,6-di-t-butyl-p-phenylphenol,2,2′-methylenebis(4-methyl-6-t-octylphenol),4,4′-butylidenebis-6-t-butyl-m-cresol,tetrakis[methylene-3-(3′-5′-di-t-butyl-4′-hydroxyphenyl)propionate]methane,1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene,n-octadecyl-β-(4′-hydroxy-3′,5′-di-t-butylphenyl)propionate, 2-n-octylthio-4,6-di(4′-hydroxy-3′,5′-di-t-butyl)phenoxy-1,3,5-triazine,4,4′-thiobis-[6-t-butyl-m-cresol],2-(2′-hydroxy-3′-t-butyl-5′-methylphenyl)-5-chlorobenzotriazole.

In addition to these components, the lubricating oil may contain extremepressure agents, viscosity index elevating agents, wear inhibitors, etc.which may be well known ones.

To further illustrate the present invention in greater detail, thefollowing Examples and Comparative Examples will be given.

EXAMPLES 1 TO 11 AND COMPARATIVE EXAMPLE 1

As Tables 1 to 3 show, definite additives (antioxidant, oily agent,etc.) were added to various base oils to thereby give lubricating oils.In these tables, each value given in brackets means the content (% byweight) of the component based on the whole lubricating oil. Theadditives were added so as to give a total content of 5% by weight basedon the lubricating oil.

Then each lubricating oil was injected into the bearing space of asample bearing [single row, deep groove ball bearing; non contact steelseal (model Z); No. SR1810, 7.94 mm (inner diameter)×12.7 mm (outerdiameter)×3.97 mm (width)] to give the content (% by volume) specifiedin Tables 1 to 3. The lubricating oil was sealed by preliminarilyputting a sealing member into the lower part of the bearing, injectingthe lubricating oil in the definite amount, and then fixing the sealingmember to one end. As the cage, use was made of one containing the samelubricating oil as the injected one in the amount as specified in Tablesby immersion.

Each sample bearing thus formed was subjected to various tests as willbe shown hereinbelow. Tables 1 to 3 also show the results of thesetests.

(1) Rocking-tolerance Test

This test was performed under the following conditions to evaluate thefretting wear resistance and durability (life) of each bearing.

rocking frequency: 30 Hz

outer ring rocking angle: 8°

axial load: 29.4 N

number of repeated rocking: 5,000,000

atmospheric temperature: ordinary.

The rocking resistance was evaluated in the following manner. After thecompletion of the rocking durability test, each test bearing wasdecomposed and the conditions of the inner ring, outer ring and cagewere observed. Samples showing no wear in the inner ring racewaysurface, outer ring raceway surface and cage were expressed in “∘” andthose showing running traces in the inner ring raceway surface and outerring raceway surface were expressed in “Δ”. These samples were referredto as coming up to the standard. On the other hand, those showing wearin the inner ring raceway surface and outer ring raceway surface wereexpressed as “x” and referred to as failing to come up to the standard.

(2) Oil Leakage Test

Each sample bearing containing the lubricating oil injected therein wasallowed to stand at 25 for 30 days and it was confirmed under astereo-microscope whether the oil leaked out or not. Samples showing noleakage were expressed in “∘” while those showing leakage were expressedin “x”.

(3) Dusting Test

In a sealed container, the outer ring of each sample bearing was rotatedat 7,200 rpm and the dusts thus formed were counted with an out particlecounter. Dusts of 0.1 μm or above in particle size per 0.1 cf (cubicfoot) were counted. Samples showing 150 or less dusts were expressed in“∘” and referred to as coming up to the standard. On the other hand,those showing more than 150 dusts were expressed as “x” and referred toas failing to come up to the standard.

(4) Torque Test

The inner ring of each sample bearing was rotated at 2 rpm under anaxial load of 9.8 N. Then the torque required at the initial stage wasmeasured and the change in the torque was monitored. Samples requiringan initial torque of not more than 1.0 gf.cm and showing a change intorque of 0.2 gf.cm or less were expressed in “∘” and referred to ascoming up to the standard. On the other hand, those exceeding theselevels were expressed as “x” and referred to as failing to come up tothe standard.

TABLE 1 Example Example Example Example 1 2 3 4 Base oil ester esterester ester oil [95] oil [95] oil [95] oil [95] Kinematic viscosity 3232 32 32 of base oil [mm²/s 40° C.] Amount of injected 1 1 30 30lubricant oil [vol %, based on bearing space) Oil content in cage 0.1578 0.15 78 [wt. %] Rocking resistance Δ Δ ∘ ∘ Oil leakage ∘ ∘ ∘ ∘Dusting ∘ ∘ ∘ ∘ Torque ∘ ∘ ∘ ∘

TABLE 2 Example Example Example Example 5 6 7 8 Base oil [95] esterester ester ester oil [95] oil [95] oil [95] oil [95] Kinematicviscosity 298 298 298 298 of base oil [mm²/s 40° C.] Amount of injected1 30 1 30 lubricant oil [vol %, based on bearing space] Oil content incage 0.15 0.15 78 78 [wt. %] Rocking resistance Δ ∘ Δ ∘ Oil leakage ∘ ∘∘ ∘ Dusting ∘ ∘ ∘ ∘ Torque ∘ ∘ ∘ ∘

TABLE 3 Example Example Example C. Example 9 10 11 1 Base oil esterester ester Li grease oil [95] oil [90] oil [85] PAO [15] ether mineraloil [5] oil [10] Kinematic viscosity 50 150 250 100 of base oil [mm²/s40° C.] Amount of injected 10 30 20 10 lubricant oil [vol %, based onbearing space] Oil content in cage 15 30 20 10 [wt. %] Rockingresistance ∘ ∘ ∘ ∘ Oil leakage ∘ ∘ ∘ ∘ Dusting ∘ ∘ ∘ ∘ Torque ∘ ∘ ∘ x

As Tables 1 to 3 show, the test bearings of the examples of the presentinvention are each excellent in all of the items of rocking resistance,oil leakage, dusting and torque properties.

For comparison, the same tests were performed on a sample bearing havingan Li grease lubricant injected into the bearing space to amount 10% byvolume. As a result, this sample bearing achieved almost comparableresults to those of the invention products in rocking resistance, oilleakage and dusting but inferior thereto in torque properties. Thissample bearing is shown as Comparative Example 1 in Table 3.

EXAMPLES 12 TO 18 AND COMPARATIVE EXAMPLE 2

As Tables 4 and 5 show, definite additives (antioxidant, oily agent,etc.) were added to various base oils to thereby give lubricating oils.In these tables, each value given in brackets means the content (% byweight) of the component based on the whole lubricating oil.

Then each lubricating oil was injected into the bearing space of asample bearing [single row, deep groove ball bearing; non contact rubberseal (model V); No. SR695, 5 mm (inner diameter)×13 mm (outerdiameter)×4 mm (width)] to give the content (% by volume) specified inTables 4 and 5. The lubricating oil was sealed by preliminarily puttinga sealing member into the lower part of the bearing, injecting thelubricating oil in the definite amount, and then fixing the sealingmember to one end. As the cage, use was made of one containing the samelubricating oil as the injected one in the amount as specified in Tablesby immersion.

Each sample bearing thus formed was subjected to various tests as willbe shown hereinbelow. Tables 1 to 3 also show the results of thesetests.

(5) Bearing Sound Test

This test was performed under the following conditions to evaluate thehigh-speed rotary performance and durability (life) of each bearing.

bearing rotation rate: 12,000 rpm (outer ring)

axial load: 2 kgf

atmospheric temperature: 90° C.

The sound was measured by using an Andelon meter. The Andelon valueimmediately after the injection of the lubricating oil (initial Andelon)was compared with the Andelon value after continuously operating for2,000 hours. Table 4 shows the results. Samples showing a ratio of 2.5or less were referred to as coming up to the standard.

¤: 0.5 or less

⊚: 0.6-1.0

∘: 1.1-2.5

Δ: 2.6-5.0

x: more than 5.1.

(6) Oil Leakage Test

Each sample bearing containing the lubricating oil injected therein wasallowed to stand at 25° C. for 30 days and it was confirmed under astereo-microscope whether the oil leaked out or not. Samples showing noleakage were expressed in “∘” while those showing leakage were expressedin “x”.

(3) Dusting Test

In a sealed container, the outer ring of each sample bearing was rotatedat 7,200 rpm and the dusts thus formed were counted with an out particlecounter. Dusts of 0.1 μm or above in particle size per 0.1 cf (cubicfoot) were counted. Samples showing 150 or less dusts were expressed in“∘” and referred to as coming up to the standard. On the other hand,those showing more than 150 dusts were expressed as “x” and referred toas failing to come up to the standard.

TABLE 4 Example Example Example Example 12 13 14 15 Base oil DOS DOS DOSPAO [97.45] [97.45] [97.45] [85.95] Additive [2.55] [2.55] [2.55][14.05] Kinematic 12 12 12 48 viscosity of base oil [mm²/s 40° C.]Amount of 3 3 3 25 injected lubricant oil [vol %, based on bearingspace] Oil content in 0.1 1.2 40 0.1 cage [wt. %] Sound property ∘ ⊚ ∘Oil leakage ∘ ∘ ∘ ∘ Dusting ∘ ∘ ∘ ∘

TABLE 5 Example Example Example C. Example 16 17 18 2 Base oil DOS DOSDOS DOS [18.09] [54.24] [16.32] [34.38] PAO POE POE POE [72.36] [36.16][10.88] [51.57] PAO [63.25] Additive [9.55] [9.6] [9.55] [14.05]Kinematic 100 70 150 70 viscosity of base oil [mm²/s 40° C.] Amount of25 35 50 55 injected lubricant oil [vol %, based on bearing space] Oilcontent in 3 3 3 0.1 cage [wt. %] Sound property ⊚ Oil leakage ∘ ∘ ∘ xDusting ∘ ∘ ∘ x

As Tables 4 and 5 show, the test bearings of the examples of the presentinvention are each excellent in all of the items of sound property, oilleakage and dusting properties. In contrast thereto, the sample bearingof Comparative Example 2 achieved almost comparable results to those ofthe invention products in sound property, but inferior thereto in oilleakage and dusting properties, since the lubricating oil had beeninjected thereinto in an amount exceeding the level as defined in thepresent invention.

As discussed above, in the rolling bearing for HDDs of the presentinvention, films made of a lubricating oil are formed, as a substitutefor the greases employed in the conventional cases, on the surface ofthe inner ring and outer ring raceway surfaces, rolling elements andcages to achieve lubricating effect. Compared with greases, lubricatingoils are highly flowable and free from any increase or change in torqueas observed in the case with the use of greases. Thus, the rocking orhigh-rotary motion can be facilitated thereby. In addition, there arisesno problem of the local insufficient lubrication due to the lubricantmasses scraped out from the contact face of the inner ring and outerring raceway surfaces and rolling elements, thus establishing a highresistance to fretting wear. Since the lubricating oil is injected intothe bearing space to as to amount to 1 to 50% by volume of the bearingspace, the lubricating effect would not disappear early.

While there has been described in connection with the preferredembodiment of the invention, it will be obvious to those skilled in theart that various changes and modifications may be made therein withoutdeparting from the invention, and it is aimed, therefore, to cover inthe appended claim all such changes and modifications as fall within thetrue spirit and scope of the invention.

What is claimed is:
 1. A rolling bearing for a hard disk drivecomprising: an inner ring; an outer ring; a plural number of rollingelements located between said inner ring and said outer ring; a cagesupporting said plural number of rolling elements; a pair of sealingmembers fixed to both ends in an axial direction of one of said innerring and said outer ring and disposed opposite to each other; and a solelubricant consisting of lubricating oil directly injected into ato-be-sealed bearing space defined between said sealing members at theboth ends in the axial direction, wherein the amount of the lubricatingoil is in a range of 1 to 50% by volume of the to-be-sealed bearingspace, in which an amount of said lubricating oil is preliminarilycontained in said cage, and the amount is in a range of 10-40% by weightof said cage.
 2. The rolling bearing for a hard disk drive according toclaim 1, in which the amount of said lubricating oil is not more than30% by volume of the bearing space.
 3. The rolling bearing for a harddisk drive according to claim 1, in which the amount of said lubricatingoil is in a range of 4-25% by volume of the bearing space.
 4. Therolling bearing for a hard disk drive according to claim 1, wherein apredetermined amount of said lubricating oil is injected into theto-be-sealed bearing space of the rolling bearing while said lubricantoil is prevented from adhering to an external portion of the rollingbearing.
 5. A rolling bearing for a hard disk drive comprising: an innerring; an outer ring; a plural number of rolling elements located betweensaid inner ring and said outer ring; a cage supporting said pluralnumber of rolling elements; a pair of sealing members fixed to both endsin an axial direction of one of said inner ring and said outer ring anddisposed opposite to each other; and a sole lubricant comprising alubricating oil directly injected into a to-be-sealed bearing spacedefined between said sealing members at the both ends in the axialdirection, wherein the amount of the lubricating oil is in a range of 1to 50% by volume of the to-be-sealed bearing space, in which an amountof said lubricating oil is preliminarily contained in said cage, and theamount is in a range of 10-40% by weight of said cage and wherein thekinematic viscosity of the sole lubricant is not more than 400 mm²/s. 6.A hard disk drive spindle device comprising: a spindle device; and arolling bearing for said spindle device, comprising: an inner ring; anouter ring; a plural number of rolling elements located between saidinner ring and said outer ring; a cage supporting said plural number ofrolling elements; a pair of sealing members fixed to both ends in anaxial direction of one of said inner ring and said outer ring anddisposed opposite to each other; and a sole lubricant consisting oflubricating oil directly injected into a to-be-sealed bearing spacedefined between said sealing members at the both ends in the axialdirection, wherein the amount of the lubricating oil is in a range of 1to 50% by volume of the to-be-sealed bearing space, in which an amountof said lubricating oil is preliminarily contained in said cage, and theamount is in a range of 10-40% by weight of said cage.
 7. The hard diskdrive spindle device according to claim 6, in which the amount of saidlubricating oil is not more than 30% by volume of the bearing space. 8.The hard disk drive spindle device according to claim 6, in which theamount of said lubricating oil is in a range of 4-25% by volume of thebearing space.
 9. The hard disk drive spindle device according to claim6, wherein a predetermined amount of said lubricating oil is injectedinto the to-be-sealed bearing space of the rolling bearing while saidlubricant oil is prevented from adhering to an external portion of therolling bearing.
 10. A rolling bearing for a hard disk drive comprising:an inner ring; an outer ring; a plural number of rolling elementslocated between said inner ring and said outer ring; a cage supportingsaid plural number of rolling elements; a pair of sealing members fixedto both ends in an axial direction of one of said inner ring and saidouter ring and disposed opposite to each other; and a lubricating oilcontained in a sealed bearing space defined between said sealing membersat the both ends in the axial direction, wherein the amount of thelubricating oil is in a range of 1 to 50% by volume of the bearingspace, in which an amount of said lubricating oil is preliminarilycontained in said cage, and the amount is in a range of 10-40% by weightof said cage.
 11. A rolling bearing for a hard disk drive comprising: aninner ring; an outer ring; a plural number of rolling elements locatedbetween said inner ring and said outer ring; a cage supporting saidplural number of rolling elements; a pair of sealing members fixed toboth ends in an axial direction of one of said inner ring and said outerring and disposed opposite to each other; and a lubricating oilcontained in a sealed bearing space defined between said sealing membersat the both ends in the axial direction, wherein the amount of thelubricating oil is in a range of 1 to 50% by volume of the bearingspace, in which and amount of said lubricating oil is preliminarilycontained in said cage, and the amount is in a range of 10-40% by weightof said cage and wherein the kinematic viscosity of the lubricating oilis not more than 400 mm²/s.
 12. A hard disk drive spindle devicecomprising: a spindle device; and a rolling bearing for said spindledevice, comprising: an inner ring; an outer ring; a plural number ofrolling elements located between said inner ring and said outer ring; acage supporting said plural number of rolling elements; a pair ofsealing members fixed to both ends in an axial direction of one of saidinner ring and said outer ring and disposed opposite to each other; anda lubricating oil confined to a sealed bearing space defined betweensaid sealing members at the both ends in the axial direction, whereinthe amount of the lubricating oil is in a range of 1 to 50% by volume ofthe bearing space, in which an amount of said lubricating oil ispreliminarily contained in said cage, and the amount is in a range of10-40% by weight of said cage.
 13. A rolling bearing for a hard diskdrive comprising: an inner ring; an outer ring; a plural number ofrolling elements located between said inner ring and said outer ring; acage supporting said plural number of rolling elements; a pair ofsealing members fixed to both ends in an axial direction of one of saidinner ring and said outer ring and disposed opposite to each other; anda lubricating oil injected into a bearing space defined between saidsealing members at the both ends in the axial direction, wherein theamount of the lubricating oil is in a range of 1 to 50% by volume of thebearing space, wherein said lubricating oil is preliminarily containedin said cage and an amount of said lubricating oil is preliminarilycontained in said cage, and the amount is in a range of 10-40% by weightof said cage.